Since the author guidelines say to follow examples as provide in New Engl J Med, I would try that output style?

10. References: References should be numbered consecutively in the order in which they are cited in the text. The format of the references in the bibliography section should conform with the examples provided in New Engl J Med 1997;336:309-15.

References with four or more authors, the first three authors only must be listed, followed by et al. EExamples of the correct form for references: